Medium Sudoku Puzzles are ideal for intermediates or beginners who are craving for more after the easy levels stop being a challenge. Logic will still prevail to solve these puzzles, but you must work harder to reach your goal.
The rules remain unchanged at this level as you still must fill the cells with numbers from 1 to 9 without any repetitions per column, row or group. However, there will be less allocated digits at the start of the game, making it more challenging to find the correct position of the missing ones. Sifting through the grid will no longer be enough and notes and tactics become more important.
The techniques to solve medium Sudoku levels are all based on pencil notes. They can be used to either find the solution for a cell or to eliminate candidates and remove unnecessary information from the grid. There are 4 main techniques to achieve this. In Sudoku jargon these are called naked single, hidden single, naked pair and hidden pair and they all require you to note down every single candidate for each cell in order to apply them.
You have a naked single when you find one block with one single possible candidate. In that case, you have found the solution for that cell, regardless if that digit is also a candidate for other blocks within the same row, column or group. Hidden singles follow the same logic but backward. In this case, you can have several candidates per cell, but one digit has only a single possibility within that row, column or group. It is called “hidden” because other candidates create noise making it harder to spot.
With the pairs techniques, you can only eliminate candidates.
When you have the same pair of candidates in a single row or column you have found a naked pair. At this point, you can be certain of their overall position in the grid, even if you cannot yet tell the final position of each digit. This allows you to eliminate those candidates from any other possibility within that row or column.
Hidden pairs occur within a group and, once again, they are hidden because they share their cells with other candidates. The principle is simple. If within the group you have two identical pairs whose digits are not candidates in any other cell of that group, then those cells will be their final position even if the solution is still unknown. Thus, you can eliminate the remaining candidates that share the cells with those numbers.